1. Field of the Invention
The present invention relates to a method of correcting image errors in the transmission of difference pulse code modulated (DPCM) coded image signals.
2. Description of the Prior Art
In the transmission of image signals, two fundamental methods are known for correcting image errors, the origin of which consists of disturbances on the transmission link. In the case of the first method, error-correcting codes are used to reduce the transmission errors. In order to be effective, however, these require a component of more than 5% of the overall transmission capacity. In the case of the second method, so-called image error masking is used. For this purpose, it is merely necessary to recognize a transmission error and to replace the faulty image signal values (PCM or DPCM coded scan values) by correction values which have been obtained from a preceding television image or have been obtained by interpolation from the surrounding image signal values. If the correction values are obtained from a preceding television image/television half image, an image memory/half image memory is required at the receiving end. When moving images are to be transmitted, these correction values can themselves result in errors.
If image signals are transmitted by way of difference pulse code modulation (DPCM) coded signals, an error in one single DPCM value will be sufficient to produce a so-called error trail which results in a considerably greater disturbance in the television image than one single, faulty image signal value such as occurs in transmission of pulse code modulated (PCM) signals.